


Fins and Frills.

by RussianSunflower3



Series: Sunflower's WHUMPTOBER 2018 [24]
Category: Haikyuu!!
Genre: 1 lewd joke, Drowning, M/M, Magic, Whumptober 2018, mermaid au
Language: English
Status: Completed
Published: 2018-10-24
Updated: 2018-10-24
Packaged: 2019-08-06 21:16:14
Rating: General Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,466
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16395245
Author URL: https://archiveofourown.org/users/RussianSunflower3/pseuds/RussianSunflower3
Summary: Kunimi, young Prince of a mermaid Kingdom, had been warned never to breach the surface.But he's interested, curious even, and rules aren't going to stop him.





	Fins and Frills.

_“Never go to the surface, Akira”_.

Ever since the mermaid Prince had been able to comprehend words, that is what he has been taught. As a mermaid, and _especially_ as a Prince, Kunimi knows he should heed those words.

But then, his parents had also told him too much salted seaweed would turn his tail green. His tail was still the same iridescent silver as before.

So his parents were liars.

“Akira, you have to wake up.”

“Go away, Tobio. It’s my birthday, I’m sleeping in.” Kageyama was the only mermaid his age within the castle, and therefore, the closest he had to a friend. With a tail as blue as the ocean deep, the edges of his scales lighter in colour to match his eyes, Kageyama was of high status.

Unfortunately for Kunimi, that meant he was constantly there to bother him.

“But the King and Queen have requested your presence!” With a groan, Kunimi rolls over, trying to tug his kelp blankets back over him.

“Akira!!!” No such luck. Irritated, Kunimi swims from his bed over to the open window and straight out it, Kageyama shouting at him to take the proper route, and takes the shortcut down to the hallway just outside the main hall. 

Matsukawa, one of the guards with lionfish markings in dark green and indigo, doesn’t even jump, just lazily raising a thick brow.

“Causing trouble so early, Young Prince?” Kunimi scoffs, settling himself on the window edge and playing with the tendrils of the underwater plants that frame each window.

“Apparently my parents have requested my audience.”

“Ah, that time of year again~.” Matsukawa holds back a chuckle at the sharp glare sent his way, knowing just how much Kunimi despises his birthday, and the official traditions that come with it. Matsukawa hums in fake thought as he leans against his trident.

“You’re 15 now though. That means you get to _choose_ one of your presents.” At those words, Kunimi perks up, fingers pausing in twisting flowers round to make a crown.

“I can choose?”

“Whoops, I probably wasn’t supposed to let that slip.”

“But you did. So tell me.” With a ghost of a laugh, Matsukawa pushes his fringe back, and gently helps Kunimi to fit the flower crown into place.

“Well… 15 is like, a quarter of a quarter through your life, right?”

“An eighth?”

“Yeah, that. So basically, ahhh… How to explain this? Every time you reach one of those ‘eighth’ milestones, you get to choose a present. Anything you want.” Kunimi’s eyes light up, his tail flicking from side to side in a pleased manner, creating a spiral of bubbles.

“Anything I want…” 

“Akira!” He loses all his enthusiasm with a sigh that’s half a growl, rolling his eyes as Kageyama comes around the corner. Matsukawa smiles at him to give him a little encouragement, then swims asides to let the two through the doors.

The doors, made from shipwreck wood, are heavy to open, but Kunimi and Kageyama easily push open one each, and swim patiently over to the giant oyster table, the King and Queen already waiting. 

“Mother, Father.” Kunimi bows to them in respect, keeping his fin low. The King gestures for him to sit, and he floats elegantly onto the moss covered protrusion from the floor. Kageyama waits by a wall in the background, as his duty states.

“You’re 15, Akira.”

“Yes.”

“We have many celebrations planned today! A ball, the chance to meet other royal families across the ocean, presents a plenty, and a chariot ride around the kingdom!” Kunimi remains quiet, biting his tongue, and looking hopefully up at his parents.

He desperately wants them to mention the present he gets to _choose_.

“Now, since you are 15, there is a slight difference this birthday.”

“You many choose one present, anything you’d like-” Interrupting may be rude, but Kunimi doesn’t wait for his mother to finish.

“To be left alone. I want…. To be left alone. All day.” His parents blink at him in astonishment, share a glance, and then look back to him.

“Akira, I’m not sure that’s… Wise…”

“It would be unsafe!” 

“You said I could have _anything_. And I want to be left alone. _Please_! I never get time alone! I just want some time to myself!” His Mother sighs, rising from the table to swim to his side, pearlescent tail pressing comfortingly against his back.

“Because you’re a Prince, Akira… We can’t lose you, not as parents, not as a Kingdom.” Kunimi’s eyebrows furrow with anger, and he bristles, the scales on his tail becoming jagged and darkening.

“I never asked to be a Prince! Put Tobio as the Prince, or something! I just want a day to myself!” 

“Enough!” The king, with a tail as black as the ocean trenches, glittering with the colours of a neon tetra, slams his fist on the table.

“You will _not_ go alone. You may have the day to yourself, but you are to take protection with you.” Kunimi rolls his eyes, but thinks carefully.

Who would be least likely to stop him doing something reckless? Who would endorse curiosity, discovery, and mischief? Who is the most likely to let him break a rule or two…?

“Hanamaki-san. I’ll take Hanamaki, from the long range Palace guard.” The answer seems to placate his Father, who sends a messenger to retrieve Hanamaki from the very top of the castle. He’s usually up there whilst on duty, captain of the archers who defend the shell-shaped palace from intruders, sharks, orca, and other dangers.

But for today, it seems he’ll get a break.

Or, at least a break from the shell tower. He’ll still effectively be guarding something.

And _that’s_ how Kunimi gets himself a whole day of freedom, weaving through kelp forests, exploring shipwrecks, riding ocean currents, all with Hanamaki by his side.

With a solid brown tail, Hanamaki doesn’t look that impressive. Until they reach a patch of ocean sunlight filters through. The sun brings out pink koi-like patterns, some remaining brown, others a mix between the two, some as pink as the anemones amongst the coral.

“Oi, Kunimi, that’s far enough.” Kunimi stops swimming mostly out of shock, looking back to see Hanamaki has paused just _meters_ from the surface.

He can see the sun, he can see the rippling of the waves above, he can hear things that he’s never heard before - strange creatures he’s read are ‘birds’ and ‘humans’ - coming from the shallow waters and surface nearby.

“Wh-What, no. I want to see.”

“You’re not allowed above the surface. No one is allowed above the surface.” Kunimi looks at him with such heartbreak and disappointment, tail losing all iridescence and becoming a dull silver, that Hanamaki sighs, rubbing the back of his head.

“Look, I know you only chose me because I let you break a _few_ rules here and there, but… This one is a biggie.” 

“... It’s just one more rule…” 

“One more rule that could get me executed and you punished for eternity.” Seeing that Hanamaki is deadly serious, and glancing cautiously back towards the Kingdom as if someone is going to come for them, Kunimi sighs and slowly swims back to him.

Hanamaki greets him with a sad, gently smile, resting a hand on the back of the disheartened Prince’s head, until Kunimi pulls away feeling a little better.

“Can we still go to the underwater cave maze?”

“Oh fins, you are gonna _love_ the cave maze!” Kunimi quirks a smile, ready to follow after Hanamaki, when he hears a disturbance. There’s splashing. It’s coming from… From the very shallow shore. Where the sunlight makes the water look clear.

“What…?” He’s drawn to it like a fish to a hook, approaching with caution, until a hand wraps around his wrist. Hanamaki tries to tug him back, but Kunimi squints, thinking he can see something in the water.

“What are those, Hanamaki-san?”

“A sign that we’re _far_ too close.”

“But what _are_ they?”

“... Legs. Strange creatures called humans have them.” Kunimi makes a soft sound of awe. He’s read about humans, but he never knew legs looked like that. They’re so… weird. It’s like they’re wrapped in kelp, but… It’s not kelp. It’s a strange thing called ‘clothing’. 

“Kunimi, come _on_. Humans are dangerous. If they’re getting in the water, we need to leave.”

“But I don’t think it’s coming in?” The legs of the human kick frantically, scrambling to get purchase on what looks like a wooden beam, supporting something Kunimi has read is a ‘pier’, constantly slipping and struggling to get back up. Hanamaki watches with the same curiousity, concern rising.

“Stay here.” He plants his hand firmly on Kunimi’s chest to indicate he remain _exactly_ where he is, and then swims closer. Hanamaki would never let the Prince do this, and he’d be executed if he did, but…

As long as Kunimi keeps it a secret, he’ll break the rule to keep his Kingdom safe. If this is some kind of weapon or tactic the humans are developing, he needs to destroy it as soon as possible. Moving under the pier, Hanamaki creeps forwards and cautiously breaks the surface, pupils thinning to slits against the light.

“No, no! Please stop! Please! I can’t swim, I _can’t swim_! Stop, please!” Hanamaki hears the tearful begging, and then cruel laughter from above, from on top of the pier. He peers between the planks and spots three human children.

“You’re so lame, Fisherman’s son~! You can’t even _swim_?”

“Haha! You should rot on the seabed like he did!” 

“No, please, no, no, no, I can’t swim, please don’t push me in!” The legs start to kick again, and for a moment, Hanamaki spots the human’s feet before there’s a yelp and he’s plunged back in to hip-depth. Hanamaki’s frills quiver with pure _anger_ when he realises the human child is being _forced off_ the pier by the other three.

Cruel. Humans are cruel. Even to their own kind.

Diving back down, Hanamaki swims back to Kunimi, his voice tight and deep with the rage that boils inside him.

“Let’s go.”

“Is it dangerous?”

“...”

“Then I want to see too.” Hanamaki sighs with agitation, twisting Kunimi around by the shoulders.

“ _Maze_. Swim.” Pouting, Kunimi starts off that way, but then they both hear the splash. Looking back in synchronization, they spot a human figure slowly descending in the waters, no matter how much it - _he_ \- tries to swim back up.

_“I can’t swim!”_ Hanamaki makes his decision quickly.

“Kunimi, head to Oikawa and Iwaizumi’s home, _now_! Ask them for the shifting potion!”

“Wha-”

“NOW!” Kunimi flinches at first, but recognising how serious the situation is, he swims off as fast as he possibly can, like a mako shark through the water, towards a rock cave a little away from the main village.

In this home, lives Oikawa and Iwaizumi. The former is a sea witch, brewing up potions for request, or for fun. The latter, his long-suffering partner, who organises and catalogues all the potions, making deliveries.

Kunimi barely stops in time to knock, although he supposes his body slamming against the boulder door sufficed.

“Come in~.” Not even affected by the interruption, Oikawa’s calm voice comes through from deeper in the cave, and the boulder door rolls open as he pulls the current simply by waving a hand. 

He’s scarily powerful.

But Kunimi is not afraid. In sorts, he’s befriended the happy couple, often stopping by to listen to Oikawa’s stories, or learn to read human books from Iwaizumi. How the mermaid knew how to read in the first place remained a mystery, but Kunimi wasn’t going to turn down forbidden education.

“I need a shifting potion.” Oikawa’s hand stills over his ‘cauldron’, a dip carved out in the cave floor. As soon as he registers the words, a sly smirk grows on his face and he sprinkles the ingredient into his current mixture.

“Oh~? And where did young Kunimi-chan learn about that, hmm?”

“Hanamaki ordered me to get it. A human fell in the water.” Oikawa’s eyes widen, and he instantly sweeps his speckled brown tail around, emptying out the mixture in the cauldron. In a flurry, he moves about his shelves of ingredients, pulling off things that he’ll need and chucking a few towards Kunimi to catch.

“How long ago was this?”

“I- I came straight here. Maybe two minutes?”

“Did you see the human? How old was it?”

“Umm… Not clearly… But I think it was about my age.”

“Makes sense… Makki wouldn’t rush into danger if it was an adult…” Kunimi lifts an eyebrow, tilting his head a little as he holds the precious vials close, Oikawa occasionally taking one back to throw something in the cauldron, mixing it with items he’s picked up.

The wriggling starfish arm makes Kunimi squirm uncomfortably.

“Why does all this… Matter?” Oikawa’s face is void of his usual cheer, focusing entirely on mixing the potion, expertly measuring what he needs with just his fingertips.

“A shifting potion, little one, is one of the hardest potions to make. I need to be precise with who I’m making it for.” 

“Was my information enough?”

“Thankfully, yes. Oh, I also peered into your mind a little~.” Oikawa grins, giving him a peace sign, and Kunimi rolls his eyes. He watches as the potion turns from a lumpy green, to cloudy blue, to a viscous golden liquid. 

“There we go. Let me bottle this and then we’ll be on our way.” As Kunimi waits for Oikawa to find an empty vial, the boulder door moves.

“I’m home- Oh, Kunimi. Hello.” Iwaizumi seems a little surprised at the Prince’s untimely visit, but then huffs a laugh at the sight of him swamped with ingredient vials. 

“Honestly, that Dummykawa. Let me help you.”

“Thank you, Iwaizumi-san. Ah, welcome home.” Iwaizumi drops off his delivery bag, ruffling Kunimi’s hair with a grin, and then starts putting the vials back. When he catches sight of the golden mixture in the cauldron, he freezes.

“... That’s a shifting potion.”

“Y-Yes? I asked for it.” Iwaizumi whips his gaze around and it’s _fierce_ , but there’s something vulnerable behind it.

“This had better not be for you!” Stunned by the sudden anger, Kunimi shrinks away a little, shaking his head. Iwaizumi mutters under his breath, and Kunimi can’t help but notice that his hand drifts over his dull, plain black tail.

It’s not a pretty tail, not like everyone else’s in this Kingdom. It doesn’t change with light like Hanamaki’s, have striking patterns like Matsukawa and Oikawa’s, and it doesn’t shimmer like Kunimi’s.

Iwaizumi’s tail, quite frankly, is ugly in mermaid standards.

Oikawa comes back through from the back room with an empty glass bottle and stills at the sight of Iwaizumi, who glares at him intently. For a second, Kunimi thinks they’re going to start fighting, but then Iwaizumi sighs, gesturing for Kunimi to sit with him whilst Oikawa bottles the potion and infuses it with magic.

“Kunimi, do you know what a shifting potion is?”

“I can guess from the name. It changes one thing to another?”

“Permanently, yes.” Another sigh, and Kunimi worries he’s done something wrong, but then Iwaizumi smiles, watching Oikawa’s back with so much love in his eyes that Kunimi feels he’s intruding on a moment.

“A shifting potion changes humans into mermaids and vice versa. When- When Oikawa and I were 17… I was human.” Kunimi’s eyes _fly_ open, as wide as they physically can. Iwaizumi laughs, ruffling his hair.

“I know! Roughly 200 years ago now! See, I was a freestyle diver, and Oikawa was a compulsive rule-breaker. We kept meeting by the coral reef, right on the edge of the territory~...” His smile fades into a frown, and his eyes darken.

“But then my parents split. My dad… He hurt me. Badly. I only escaped because I swam away, in the dead of the night, with a storm brewing. Oikawa must have sensed something was wrong, because I met him at the reef.”

“Iwa-chan was, undoubtedly, going to die.” Kunimi briefly switches his attention to Oikawa before whipping his head back to Iwaizumi with disbelief. He was a human? He almost _died_?

“Mhm~. Bleeding and unable to breathe, I started to drown. Oikawa dragged me to the surface, but there was water in my lungs. That’s when… When he brought me the shifting potion.”

“It was the first one I ever made successfully! But for it to work, the person drinking it has to desperately want to change forms.”

“Of course I did. Oikawa was a mermaid, I loved him, I wanted to be with him no matter what. Not dying was just a bonus.” Oikawa swims over with the bottled potion, glowing a bright gold with swirls of amber and white. He kisses Iwaizumi sweetly, passing the potion to Kunimi.

“Since that was my first ever potion, Iwa-chan’s tail is boring and bland - just like him! Ow!” Iwaizumi knocks his forehead with his knuckles, scowling, but then kisses the area when Oikawa whines that he was joking.

“This one is much better. The human won’t look so out of place.” Kunimi holds the bottle a little closer, feeling how cold it is beneath his fingertips. Like the deepest reaches of the ocean. Iwaizumi moves to the boulder door, gesturing out of it.

“It can be quite disorientating, so I’ll come with you to talk with the human.” Kunimi hesitates at the doorway, but Oikawa gives him a friendly shove from behind. 

“Lead the way, Kunimi-chan~.” With a curt nod, he speeds off, trusting the adults to keep up with him. He slows as he reaches the area by the docks, looking around unsure of himself.

This is where they were, right?

“Hanamaki-san…?” 

“Over here!” A flash of pink pokes out of a small crevice in the wall, and the trio head over that way. Oikawa places a hand to the rock, pressing his forehead against it.

“A cave system… One of them leads to one above ground, but it’s encased in rock. Like a bubble.” Hanamaki nods, leading them along. He’s pale and worried, and Kunimi worries the human child must have drowned. 

As they break the surface of the water, Kunimi sees what Oikawa meant. Asides from the ocean below, and a small sandbank on the side where the human lies, it’s nothing but rock. Hanamaki and Iwaizumi instantly swim towards the sand, wriggling out of the water so only their tails are dipped in. Iwaizumi presses his ear to the human’s chest.

“He’s alive. Just. But I can hear the fluid in his lungs. He’s _drowning_.” Hanamaki reaches for the shifting potion and Kunimi is about to hand it over, when Oikawa gently pulls him back.

“We have to make sure the boy _wants_ to change first. The potion won’t work otherwise.” Kunimi sinks down in the water so that just his eyes are above the water, watching as Iwaizumi pushes on the child’s chest - does _something_ \- that makes the boy cough out mouthfuls of water and gasp.

Gently, Iwaizumi runs a hand through hair that sticks straight up.

“It’s okay, kid, it’s okay~... We’ve got you, baby.” The human child lies on the sand, panting with shuddering inhales and ragged exhales. He continues to cough up little bits of water, even as Iwaizumi rubs his chest and speaks to him softly, Hanamaki’s scaled hands holding one of the child’s.

Eventually, he sits up, a little dazed and still very out of breath. He won’t survive long, with water in his lungs, unless he shifts. 

“I- Where…?” At the sight of four people around him, two of them in the water and two lying on the sand with _tails_ , the boy scrambles back against the rock wall.

“Don’t eat me, don’t eat me! I- I taste bad!” Oikawa bursts into laughter, Hanamaki snickering into an arm, and Iwaizumi looks vaguely amused.. But something about that offends Kunimi.

“We’re not going to _eat_ you, that’s stupid. It’s more likely _you_ would eat us.” Dark eyes blinks at them, wide but innocently confused.

“I… Would? But- But Mermaids eat humans! It’s the legend in our village!” Iwaizumi, this time, is the one to laugh loudly, even drowning out Oikawa’s gleeful cackle, having to submerge himself to not scare the child.

“By any chance, is that legend 200 years old?”

“... Yes?”

“Yeah, that was me. And trust me, Oikawa didn’t _eat_ me. Not in the way the ‘legend’ states anyways.” He smirks as the human child flushes bright red, and Kunimi groans in disgust, Hanamaki staring at Iwaizumi with a deadpan look.

“Why would you say that in front of a _baby_? Look at him, Iwa! He’s barely as old than Kunimi!” That piques Kunimi’s interest, but rather than move closer, he just observes, golden bottle held tightly to his chest. Iwaizumi chuckles, but then smiles softly at the boy.

“How old are you exactly, sweetheart?”

“F-Fifteen next week…” He still seems cautious, but… Something about the soft way Iwaizumi speaks, and the sweet words he uses, like talking to a much younger child, must do something to calm him.

“What were you doing in the water, hmm?” 

“The- The other kids pushed me in… They don’t like me much. But I didn’t think they’d _actually_ try drowning me…” He brings his knees to his chest and wraps his arms around them.

A bad move, he discovers, when he starts coughing up water again.

Hanamaki has longer arms than Iwaizumi, and just manages to rub the child’s back to ease him through it.

“What about your parents? Don’t they try and stop it?” 

“Don’t have any. Dad drowned on a fishing trip an’ I never had anyone else.”

“Oh, you poor thing…” Losing a little bit of tension, the boy sends a wobbly smile Iwaizumi’s way.

“It- It’s okay though! I managed five years alone! M’not gonna die soon or anything!” There’s an ominous silence. The boy gulps. He looks at the wet sand where he’s just coughed up water and pales.

“Oh god, I’m going to die. I’m going to drown above water. I’m going to drown in a _cave_ , surrounded by _**mermaids**_. I’m probably already dead!” Hanamaki rolls his eyes and gently slaps the boy’s leg.

“You’re not dead, moron, and hopefully you won’t be dying. What about becoming a mermaid instead?”

“Makki! You can’t just ask him that!”

“I, uhhhhhh…. What?” Iwaizumi slaps his forehead with a scaly hand, gesturing for the boy to move closer. It says something about the trust that’s formed between them when the human child does so, with a little hesitation.

His feet and the bottom half of his legs are in the water, and Kunimi considers pulling him in deeper by the ankles just to tease him.

“What’s your name, little one?”

“Umm… Kindaichi…”

“Is that your whole name?”

“Yuutarou, umm, Kindaichi Yuutarou.” Iwaizumi pats his shoulder in praise, wriggling back to submerge himself against briefly before coming back. 

“Now, you’re a human for the moment, just like I used to be, but there’s water in your lungs. You know what that means, don’t you?” A nod.

“There… _Is_ a way to avoid that, but… You have to want it.”

"What is it?” The four mermaids exchange glances, and Oikawa takes the golden bottle from Kunimi to hand over to Iwaizumi. The sea witch returns to Kunimi’s side as Iwaizumi explains things to Kindaichi, placing his cold hands on Kunimi’s shoulder.

“Watch~. This is a special kind of magic~.” There’s a moment of quiet, and Kindaichi crosses one ankle over the other, and closes his eyes tightly. 

“I... Don't want to be human anymore! I want to become a mermaid.” As soon as those words leave his lips, Iwaizumi pours the golden potion over his legs evenly, coating every inch. The golden liquid sinks into his skin, and then…

There’s sparks and pops and crackles and it’s glowing, and everything is so _bright_ , but so beautiful, and Kunimi can’t tear his eyes away-!

The glow dulls, and there, where there was once a human child, is a _mermaid_. Kindaichi’s hands are covering his eyes and he might be whimpering behind them, but there are scales on his webbed fingers, his ‘ears’ have been replaced with frills, and where his legs once were is a gorgeous golden tail.

Not entirely golden, either. There are rivers of amber surrounded by white all over them, patterned like a Mandarinfish but in that beautiful golden colour that reflects his name. Oikawa sniffs, proud of himself.

“I _told_ you this potion would be more extravagant!” Iwaizumi rolls his eyes, but flips back into the water to swim over and give Oikawa a kiss anyways. 

As Kindaichi gasps at his tail in awe, leaning over to pull at his fin and examine it with sparkling, excited eyes, Hanamaki waves Kunimi to his side. He wraps an arm over the Prince, talking quietly.

“Y’know, I’m glad you helped me break the rule this time.”

“Even if we’ll be executed?”

“Pfft~. I think your family might be more shocked by the strange new mermaid, but… Our cover story is that we found him crawling around the ocean floor, he was abandoned by his nomad tribe, didn’t know this was a Kingdom, and has never been taught mermaid things.”

“Well… That last point is true.” Hanamaki laughs, moving back to swim in a circle around the cave.

“Alright, kid. You’re basically a baby mermaid at this point, so we’re gonna have to help you a _lot_ until you pick it up.”

“Actually…” Oikawa interrupts, rubbing the back of his head sheepishly with a blush, arm around Iwaizumi’s waist where his scales just start to emerge.

“Iwa-chan and I have been talking about kids lately… And this seems like an ideal opportunity! We want a child, Kindaichi doesn’t have parents, _and_ he’s a new mermaid!”

“Plus I’ve been through the same situation. I can help him adjust, especially through the lashback stage.” Fortunately, Kindaichi doesn’t hear that bit, too busy admiring his new tails and talking excitedly with Kunimi about the Kingdom that will become his home. Hanamaki smiles.

“I mean, he’s gonna need a home anyways, and you guys would make _great_ parents. S’long as the King agrees, I’m all for it.” Iwaizumi cheers in success, and Oikawa happily trills, his frills flapping.

“Kindaichi! Ready for your first swim?” Kindaichi perks up, glancing between the water, his tail, and then the water again, before he slaps his fins against the water and grins at his new parents, weird uncle, and friend. He’s found a family, and a home.

“Yes!”

Who would have thought all it took was drowning?

**Author's Note:**

> Sorry it's late!!! I forgot to upload before work!  
> Please kudos and comment~.


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